Effect of a magnetic filter on hydrogen ion species in a multicusp ion source

Abstract

LBL-12255Hydrogen ion species and discharge characteristics have been compared for two different magnet geometries in a multicusp ion source. One magnet configuration indicated that the H; ion percentage in the extracted beam could be reduced by eliminating the high energy ionizing electrons near the ion extraction region. To accomplish this and maintain the desire features of both magnet geometries, a magnetic filter was installed near the source + exit. With this combined arrangement, we found that the H ion percentage and discharge condition were much improved and a more uniform profile across the extraction plane was achieved. The atomic species fraction can then be further enhanced by the addition of low energy (-16 eV) primary electrons into the source plasma chamber.

Negative ion sources for neutral beam systems

“…These cold electrons are unable to generate additional positive ions in the region of ion extraction. The use of -6-~ I this filter geometry for positive ion sources has been investigated [20] to improve the plasma density profile and to increase the atomic component of the extracted positive ion beam.…”

Further study on a magnetically filtered multicusp ion source

“…If I 1 is the ion current flowing from the source chamber into the extraction chamber, a.. is· the fraction of that current which impacts the -6-plasma grid, y is the coefficient of secondary electron emission of the plasma grid, and I 2 is the ion current lost to the anode and the plasma grid from the additional plasma generated in the extraction chamber by the secondary emission electrons, then for a first order approximation, the current collected by the grid can be expressed as: ( 1 ) The production rate of ions in the extraction chamber depends on the Thus, if a high percentage of atomic species is desired, the plasma grid should not be biased or left floating at a potential more negative than the ionization potential of molecular hydrogen (-16 eV). In addition, a very negative grid bias or floating potential will increase the power loading and the sputtering rate of the plasma grid by ions falling through the sheath.…”

“…1 The filter provides a limited region of transverse magnetic field which is made strong enough to prevent the energetic electrons in the source chamber from crossing over into the extraction chamber but it is weak enough that some plasma does diffuse into the extraction region. Since only very cold electrons are found in the extraction chamber, ionization processes are unlikely to take place.…”

Several Atomic Physics Issues Connected With the Use of Neutral Beams in Fusion Experiments

“…They are used not only for general heating, but for fueling and producing electrostatic confinement in mirror experiments. Table I accomplished by Ehlers and Leung [11] with magnetic filters in the ion source ( Fig. 1).…”